Project Summary Immune checkpoint blockade has emerged as a promising approach to treat cancer by restoring T cell effector function and breaking a tumor-permissive microenvironment. Remarkable clinical efficacy, durable response, and low toxicity of PD-1 checkpoint blockade have been observed in various malignancies including hematological cancers. However, in a phase 1 study of nivolumab (anti-PD-1 antibody; BMS-936558), none of 27 patients with multiple myeloma (MM) experienced a partial or complete response, whereas objective responses were observed in about 40% of patients with follicular lymphoma or diffuse large B cell lymphoma. As we and others have shown that MM cells express high levels of PD-L1, that bone marrow (BM)-infiltrating T cells are largely PD-1 positive, and more importantly MM cells carry somatic mutations in amounts similar to as B-cell lymphomas, the absence of response to PD-1 antibody therapy for MM cannot be explained by a lack of tumor-infiltrating T cells or PD-L1 or neoantigen expression by MM cells or immune cells. We speculated that PD-1/PD-L1 checkpoint blockade alone is insufficient to break the permissive microenvironment in MM because BM-infiltrating immunosuppressive cells, such as tumor-associated M?s, myeloid-derived suppressor cells (MDSCs), and/or regulatory T cells (Tregs) could still inhibit the function of MM-specific effector T cells restored by the checkpoint blockade. Indeed, our preliminary studies showed that, similar to human MM, PD-1 mAbs had no significant therapeutic effect against established MM in murine models. However, to our surprise, in vivo depletion of M?s, but not MDSCs or Tregs, resulted in significant anti-MM effects following PD-1 checkpoint blockade. This application will test our hypothesis is that M?s, as one of the major BM-infiltrating cell types, are crucial in suppressing T-cell immunity in the tumor microenvironment, and targeting these cells will significantly improve the therapeutic efficacy of checkpoint blockade in patients with MM. Aim 1 will determine the role and mechanism of M?s in the primary resistance to PD-1 checkpoint blockade therapy in MM. Aim 2 will elucidate the mechanisms of M?-mediated immune suppression, and Aim 3 will determine the translational potential of combining M?-targeting and PD-1 antibodies to treat human MM. Accomplishing these aims will provide the justification and tools to clinically target BM-infiltrating M?s to sensitize MM patients to PD-1 checkpoint inhibitors. The proposed studies will also lead to a better understanding of the fundamental mechanisms underlying the primary resistance to PD-1 checkpoint blockade and could pave the way to the first substantial improvements in the treatment in MM and other hematological malignancies by way of targeting M?s and PD-1 inhibition.